Device and Method for the Thermal Treatment of Fluorine-Containing and Noble Metal-Containing Products

ABSTRACT

The present invention relates to an ashing plant for enriching noble metals from fluorine-containing materials, comprising
     a thermal treatment chamber ( 1 ) having a refractory insulating lining on the inside of the chamber and   an exhaust gas cleaning system,   whereby the refractory insulating lining is resistant to hydrofluoric acid and the exhaust gas cleaning system comprises at least one thermal after-incineration chamber ( 2 ), at least one or more acid scrubber(s) ( 3, 4 ) and at least one alkaline scrubber ( 5 ).

The present invention relates to a device and a method for thermaltreatment of noble metal-containing products which also contain fluorineaside from noble metals.

Various methods have become established for recovering noble metals fromnoble metal-containing products, such as, for example, catalysts or fuelcells. In the hydrometallurgical method, the noble metal-containinglayer of catalysts is dissolved off the ceramic support by means ofstrong acids or bases. Subsequently, the noble metals are separated fromthe solution, for example through precipitation reaction.

In the pyrometallurgical method, the separation of noble metals proceedsthrough melting the noble metal-containing products in a metallurgicalprocess. The ceramic fraction is transferred in a slag phase and tapped,the noble metals are alloyed into a collector metal, which is then alsotapped and processed further.

The direct incineration of noble metal-containing sludges andmulti-element waste materials, as described in DE 31 34 733 C2 and WO99/037823, is also known. The noble metal-containing ash thus obtainedis then leached in order to recover the noble metals.

In thermal reprocessing, noble metal-containing products that alsocontain fluorine aside from the noble metals have proven to be aproblem. Thermal treatment of these products produces hydrogen fluoridegas, HF. Said gas reacts with water in the ambient air to producehydrofluoric acid.

Conventional thermal reprocessing plants, in particular ashing plants,comprise a thermal treatment chamber and an exhaust gas purificationsystem. The thermal treatment chamber is a component of a furnace and isprovided with an insulating lining made, for example, of fireclay bricksor a ramming mass. These differ in composition. However, all insulatinglinings comprise, inter alia, silicon dioxide SiO₂ (glass) and calciumoxide CaO. These components are attacked even by small amounts of thehydrofluoric acid and hydrogen fluoride that are generated and thus aredissolved out of the insulating lining which reduces the service life ofthe furnace and thus of the plant.

Another problem related to hydrogen fluoride gas is the condensation ofhydrofluoric acid at the supporting external steel shells. These are theshaping and mechanical load-bearing framework of the components of anashing plant. Hydrogen fluoride gas can diffuse through pores in theinsulating lining and, in the region of the steel shells, react withwater from the ambient air to produce hydrofluoric acid. Condensation ofhydrofluoric acid on the steel shells causes them to corrode which canrender the entire plant unstable.

A method, in which the generation of HF is to be prevented, is disclosedin EP 1 478 042 A1. In this method, components of fuel elements andcatalysts are mixed with inorganic additives. In the subsequent thermaltreatment process, the hydrogen fluorides and other fluorine compoundsare absorbed and chemically bound by the additive. For this purpose, anup to 100-fold excess of the additive is added to the hydrogen fluoridegas that is being generated. However, it has been evident that theabsorption at the additive is insufficient or too slow in the case ofmaterials releasing hydrogen fluoride already at low temperaturesallowing some hydrogen fluoride gas to escape. Moreover, the additiveoccupies a fraction of the volume of the incineration space such thatthe quantity of material that can be processed is reduced.

It is therefore the object of the present invention to provide a plantfor thermal reprocessing of noble metal-containing products that containfluorine in addition to noble metals. The plant according to theinvention shall allow all fluorine-containing products to bereprocessed, regardless of the volatility of the materials containedtherein.

Another object of the present invention is to provide a method forenriching noble metals from fluorine-containing materials.

A first embodiment meets the object on which the present invention isbased through an ashing plant for enriching noble metals fromfluorine-containing materials, comprising a thermal treatment chamber(1) having a refractory insulating lining on the inside of the thermaltreatment chamber (1) and an exhaust gas cleaning system,

whereby the refractory insulating lining is resistant to hydrofluoricacid and the exhaust gas cleaning system comprises at least one or moreacid scrubber(s) (3, 4) and at least one alkaline scrubber (5).

Another subject matter of the present invention is an ashing plant forenriching noble metals from fluorine-containing materials, comprising athermal treatment chamber (1) having a refractory insulating lining onthe inside of the thermal treatment chamber (1) and an exhaust gascleaning system,

whereby the refractory insulating lining has an aluminium oxide contentof 85% by weight or more and the exhaust gas cleaning system comprisesat least one or more acid scrubber(s) (3, 4) and at least one alkalinescrubber (5).

Moreover, the exhaust gas cleaning system according to the inventionpreferably comprises at least one or more thermal after-incinerationchambers (2). Preferably, the exhaust gas cleaning system comprises oneafter-incineration chamber (2).

FIG. 1 shows a schematic view of an ashing plant according to theinvention. The FIGURE shows a preferred embodiment that comprises twoacid scrubbers (3, 4).

The thermal treatment chamber (1) is a component of a furnace, intowhich the materials to be processed are introduced. For this purpose,the thermal treatment chamber (1) comprises an opening for introductionof the corresponding materials. According to the invention, the thermaltreatment chamber (1) can be operated with a sub-stoichiometric amountor with an excess of air. The inside of the thermal treatment chamber(1) can comprise devices for incineration of the fluorine-containing andnoble metal-containing materials. This concerns, for example, grates foraccommodation of troughs for incineration of solid materials. Liquidmaterials can be introduced into the thermal treatment chamber (1) andincinerated therein either batch-wise in troughs or continuously bymeans of corresponding dosing facilities.

The temperature on the inside of the thermal treatment chamber (1)usually is approx. 800° C. In this context, the refractory insulatinglining is designed to be stable at this continuous temperature.Moreover, it is also resistant to temperature peaks of up to approx.2,000° C. It is feasible according to the invention to heat the thermaltreatment chamber (1) directly or indirectly. All means of heating knownaccording to the prior art are feasible, for example gas and oil heatingor electrical heating.

According to the invention, an acid scrubber (3, 4) is a scrubbingstage, in which exhaust gases from the thermal treatment chamber (1) arewashed with water or with water acidified by the hydrogen fluoride gasto be washed out. According to the invention, an alkaline scrubber (5)is a scrubbing stage, in which the exhaust gases are washed with analkaline agent.

Heating fluorine-containing materials in the thermal treatment chamber(1) in the ashing plant according to the invention produces exhaustgases that contain hydrogen fluoride gas. Since the thermal treatmentchamber (1) is lined with the hydrofluoric acid-resistant insulatinglining, the chamber is not attacked by the exhaust gases. In the exhaustgas cleaning system according to the invention, the exhaust gas isinitially subjected to thermal reprocessing in a thermal after-treatmentchamber (2) and then all hydrogen fluoride gas or hydrofluoric acidalready formed is removed in the acidic and alkaline scrubbing stages(3, 4, 5) such that the exhaust gases are then harmless and can beguided to the outside, for example by means of a chimney.

The ashing plant according to the invention can provide further cleaningstages or cleaning agents for exhaust gas cleaning in order to remove,for example, soot, chlorine or nitrous gases from the exhaust gases.Pertinent cleaning agents or cleaning stages are described in the priorart.

According to the invention, the hydrofluoric acid-resistant insulatinglining is resistant both to hydrogen fluoride gas and to hydrofluoricacid.

The noble metal-containing and fluorine-containing materials are placedin the thermal treatment chamber (1). The exhaust gases produced in thethermal treatment chamber (1) during thermal treatment can first beguided into a thermal after-incineration chamber (2). Preferably, saidchamber is also provided with a hydrofluoric acid-resistant refractoryinsulating lining. Moreover, the ashing plant comprises an exhaust gasconduit (6) for guiding the exhaust gases out of the thermal treatmentchamber (1). Preferably, the inside of said exhaust gas conduit (6) isalso provided with a hydrofluoric acid-resistant refractory insulatinglining.

Thermal treatment chamber (1), exhaust gas conduit (6), and theafter-incineration chamber (2), which is preferably present, are thecomponents of the ashing plant through which exhaust gas flows beforehydrogen fluoride gas is removed from the exhaust gas in the acidic andalkaline scrubbers (3, 4, 5). Providing the thermal treatment chamber(1) as well as the after-incineration chamber (2), and the exhaust gasconduit (6) with a hydrofluoric acid-resistant refractory insulatinglining increases the service life of the ashing plant according to theinvention, since these components cannot be attacked by the hydrofluoricacid or hydrogen fluoride gas.

The refractory insulating lining of the present invention can be aramming mass. Said ramming mass preferably has an aluminium oxide(Al₂O₃) content of 85% by weight or more, in particular of 88% by weightor more. Said insulating lining is stable at a working temperature andat a continuous temperature of approx. 800° C. However, it alsowithstands peak temperatures of up to approx. 2,000° C.

Ramming masses usually contain silicon dioxide (SiO₂) and/or calciumoxide (CaO) in addition to aluminium oxide. These components are alsopresent in conventional refractory insulating linings. These aredissolved by hydrofluoric acid, which destroys the insulating lining.

Surprisingly, it has been evident that an aluminium oxide fraction of85% by weight or more, in particular of 88% by weight or more, beingpresent in a ramming mass is sufficient to provide hydrofluoric acidresistance. The ramming masses according to the invention can alsocontain different fractions of calcium oxide and silicon dioxide asfurther components in addition to aluminium oxide. Despite the calciumoxide and/or silicon dioxide fraction of the ramming mass being up to15% by weight, in particular up to 12% by weight, the ramming mass isnot attacked by hydrofluoric acid. In this context, the relative contentof calcium oxide and/or silicon oxide or their ratio with respect toeach other is irrelevant. Moreover, the corresponding ramming mass iseasy to process and easily adapts to the internal wall of the thermaltreatment chamber (1), exhaust gas conduit (6), and after-incinerationchamber (2).

In this context, it is feasible according to the invention that thethermal treatment chamber (1), the after-incineration chamber (2), andthe exhaust gas conduit (6) comprise the same or different insulatinglining(s).

Preferably, the thermal treatment chamber (1), the after-incinerationchamber (2) and/or the exhaust gas conduit (6) further comprise anexternal lining. The external lining can be provided using materialsthat are known according to the prior art. Preferably, the externallining is a mineral fibre. The external insulation is surrounded by asteel plate. The steel plate fixes the external insulation in place andserves for stabilisation and shaping of the components of the ashingplant.

One problem during the incineration of fluorine-containing products isthe generation of hydrofluoric acid that causes corrosion in the regionof the supporting external steel shells. If hydrogen fluoride gas HFdiffuses through pores to reach the space behind thetemperature-resistant insulating lining in the reaction space, itreaches the external steel constructs part of which are load-bearing.Hydrogen fluoride gas can react with water from the ambient air to formhydrofluoric acid in this location. Hydrofluoric acid forms anazeotropic mixture with water at a concentration of 38.2% HF, wherebythe boiling temperature of the azeotropic mixture is 112° C. Ifhydrofluoric acid condenses at the steel walls, it causes these tocorrode.

The effect of having the external insulation is that the temperature atthe steel shell, i.e. at the external steel wall of the plantcomponents, does not drop below 120° C. In this context, the thicknessof the external insulation is a function of the temperature profile onthe inside of the corresponding component of the ashing plant.Condensation of hydrofluoric acid does not take place at a temperatureof 120° C. Accordingly, if hydrogen fluoride gas were to diffuse throughthe refractory insulating lining towards the outside, no corrosiondamage to load-bearing steel constructs is to be expected.

According to the invention, the thickness of the insulation on thethermal treatment chamber (1), exhaust gas conduit (6), andafter-incineration chamber (2) can differ. However, it is feasible justas well that the insulation of all components is equal in thickness.Accordingly, thermal treatment chamber (1), exhaust gas conduit (6), andafter-incineration chamber (2) can, for example, comprise an externalinsulation made of a mineral fibre at a thickness of approx. 10 cm.

In a preferred embodiment, the thermal treatment chamber (1) comprises arefractory insulating lining on the inside, whereby the thickness of thewall of the chamber plus the insulating lining is approx. 30 cm, and anexternal insulation with a thickness of approx. 10 cm.

According to the invention, the ashing plant preferably comprises atleast one scrubber made of graphite (3) with a double-walled design.Said double-walled scrubber (3) preferably comprises a cooling system,in particular a water cooling system. The scrubber (3) can comprise asteel plate as external shell. The double-walled scrubber (3) comprisesvalves for feeding and discharging the coolant.

The coolant, for example water, flows between the external graphite walland the external shell. This leads to indirect dissipation of heat fromthe exhaust gas via the scrubbing medium and the graphite walls.

Water has proven to be particularly well-suited as coolant. Water isinexpensive and easy to handle. Moreover, if there was any exposure tothe exhaust gases, there would be no hazard of undesired chemicalreactions occurring.

The thickness of the graphite walls preferably is in the range of 3 cmto 4 cm. It has been evident that this thickness is sufficient tomaintain sufficient temperature and acid stability.

Water flows into the double-walled scrubber (3) as scrubbing agent forthe exhaust gas. The water reacts with hydrogen fluoride gas HF in theexhaust gas and removes it by scrubbing. The scrubbing water containingthe hydrogen fluoride gas in the form of a salt is then collected andsubjected to disposal.

Graphite is characterised not only by acid resistance, but by hightemperature resistance as well. The exhaust gases flow from the thermalafter-incineration chamber (2) or the thermal treatment chamber (1) intoan acid scrubber. The temperature of said exhaust gases is up to 1,000°C. It has been evident that the temperature resistance of graphite issufficient in this context.

Preferably, the exhaust gas cleaning system according to the inventionfurther comprises at least one single-walled scrubber (4) made ofgraphite. This scrubber also comprises a steel plate for its externalshell. Preferably, the thickness of the graphite wall is in the range of3 cm to 4 cm.

In a preferred embodiment, the exhaust gas cleaning system comprises adouble-walled scrubber made of graphite (3) and a single-walled scrubbermade of graphite (4). This embodiment is shown in FIG. 1. Both acidscrubbers (3, 4) are cylindrical in shape in this preferred embodimentand have an internal diameter of more than 1 m. The exhaust gas contactsthe scrubbing water over the entire height of approx. 4 m or more and iscleaned in the process. The scrubbing water usually flows into thescrubber from above such that the exhaust gas is washed with water inthe upper region of the scrubber. In the process, it becomes enriched inhydrogen fluoride gas to the effect that the exhaust gas is washed withan acid, namely water containing a hydrofluoric acid fraction, in thelower region. In this context, the terms, upper and lower, refer to thespatial arrangement that is also shown in FIG. 1.

In the first double-walled scrubber (3), at least a majority or all ofthe hydrogen fluoride gas is washed out of the exhaust gas with water.Moreover, the exhaust gas is also being cooled. In the secondsingle-walled scrubber (4), hydrogen fluoride gas that may still bepresent is bound and thus removed from the exhaust gas. No furthercooling of the exhaust gases is needed.

The ashing plant of the present invention further comprises an alkalinescrubber (5). The exhaust gases are guided from the at least one acidscrubber (3, 4) into said alkaline scrubber after most or all of thehydrogen fluoride has been removed. The alkaline scrubber (5) cancomprise a coating on its inside that is resistant to alkaline scrubbingwater and any traces of hydrogen fluoride gas that may still be presentin the exhaust gas. Specifically, the coating is stable when exposed tobases having a pH of at least 10 or more, in particular of at least 11or more. Preferably, the alkaline scrubber (5) comprises on its inside acoating made of a plastic material, in particular made of polypropylene.The external shell of the alkaline scrubber can consist of steel.

A plastic coating has proven to be easy to handle. The lining is made tobe homogeneous and is not associated with a risk of cracks. Moreover,plastic materials, in particular polypropylene, are resistant toalkaline scrubbing water as is used in this scrubbing stage. If anyresidual HF were still to be present at this stage, the coating wouldnot be attacked by it.

The exhaust gas guided into the alkaline scrubber (5) is largely free ofhydrogen fluoride gas. However, it cannot be excluded that some tracesof hydrogen fluoride gas may still be present. If the alkaline scrubber(5) were lined with the otherwise common glass fibre-reinforced plasticmaterials (GFR), these residual amounts of hydrogen fluoride gas wouldbe sufficient to attack and quickly etch away the glass fibres in theplastic material. The internal lining would have to be replaced afterjust a short time under these conditions.

The ashing plant according to the invention can further comprise acontrol unit. The control unit controls the temperature profiles neededduring the thermal treatment depending on the specific material and alsocontrols the exhaust air line as a function of negative pressure,temperature, and oxygen content of the exhaust gas. As a matter ofprinciple, both continuous and discontinuous operation are feasible. Adiscontinuous operation is preferred.

In a further embodiment, the present invention comprises a method forenriching noble metals from fluorine-containing materials, comprising athermal treatment of the materials in a thermal treatment chamber (1)having a hydrofluoric acid-resistant refractory insulating lining andcleaning of the exhaust gases generated during the thermal treatment,whereby the cleaning comprises the following steps in the followingorder:

-   a) if applicable, thermal after-incineration in an    after-incineration chamber (2),-   b) scrubbing of the exhaust gases with water and/or an acid, and-   c) scrubbing of the exhaust gases with a base.

A noble metal-containing ash is generated during the thermal treatmentof the noble metal-containing and fluorine-containing materials. Saidash is then reprocessed according to wet chemical methods knownaccording to the prior art in order to recover the noble metals itcontains. In the scope of the present invention, noble metals are gold,silver, and the metals of the platinum group.

The thermal treatment usually proceeds at a temperature of up to 800° C.Peak temperatures of up to approx. 2,000° C. may occur briefly.Following the introduction of the materials into the thermal treatmentchamber (1), the temperature is increased slowly up to a temperature ofapprox. 600° C. to 800° C. The specific temperature depends on thematerials to be processed.

The exhaust gases generated during the thermal treatment are firstsubjected to thermal after-incineration in an after-incineration chamber(2), if applicable. Subsequently, the exhaust gases are washed withwater or an acid in an acid scrubber. According to the invention, wateris used as the scrubbing agent. Water washes hydrogen fluoride gas outof the exhaust gas. This produces an acid, which washes out morehydrogen fluoride gas such that, in the course of the entire scrubbingprocess, both water and an acid wash the exhaust gases.

The scrubbing water can have room temperature in this context. However,it is feasible just as well that the scrubbing water has a temperaturethat is slightly higher than room temperature. In this context, thetemperature should not be more than 10 to 20° C. above ambienttemperature.

The salt-enriched scrubbing water obtained in step b) of the scrubbingprocess can be fed to a waste water treatment plant periodically orcontinuously as side stream.

Preferably, scrubbing of the exhaust gases with water and/or an acidcomprises two steps, namely

-   b1) scrubbing and simultaneous cooling of the exhaust gases in a    double-walled graphite scrubber (3) followed by-   b2) scrubbing of the exhaust gases in a single-walled graphite    scrubber (4).

It is preferred to cool the exhaust gas in the double-walled graphitescrubber (3) by means of a cooling system, in particular a water coolingsystem. Provided water is used as coolant, the temperature of the waterpreferably is approx. 60° C.

Hydrogen fluoride gas is removed from the exhaust gas by introducingwater into the inside of the double-walled scrubber (3).

In order to ensure that the hydrogen fluoride gas is removed from theexhaust gas as close to completely as possible, the exhaust gas iswashed twice with water and/or an acid in a preferred embodiment and,for this purpose, is guided from the first double-walled graphitescrubber (3) into a second single-walled graphite scrubber (4). Theexhaust gas is washed with water and/or an acid in both scrubbers (3,4). In both scrubbing stages, the initial scrubbing agent is water. Thewater becomes acidified by the hydrogen fluoride gas washed out of theexhaust gas such that, overall, both water and an acid wash the exhaustgas at the respective stages. If the hydrogen fluoride gas is alreadyremoved completely from the exhaust gas in the first stage, the secondstage entails a cleaning with water only.

In scrubbing step c), the exhaust gas is neutralised and acidiccomponents originating from step b) are removed. A base with a pH of atleast 10 or more, in particular of at least 11 or more, can be used asbase in this context. It is preferred to use sodium hydroxide solutionfor scrubbing the exhaust gases in step c). Sodium hydroxide solutiondoes not undergo any undesired reaction with the exhaust gas components.Moreover, it is inexpensive and easy to handle.

The method according to the invention is suitable preferably for theprocessing of materials that have a fluorine content of up to 5% byweight. It is preferred to use fluoro-organic materials, PTFE films,fuel cells, catalysts and/or pastes as materials to be subjected to thethermal treatment. As a matter of principle, the method is suitable forall materials having a fluorine content of up to 5% by weight thatdecompose at a temperature of approx. 800° C., in particular of approx.600° C.

It is preferred to implement the method according to the invention in anashing plant of the type described above. The ashing plant according tothe invention is suitable for the use and implementation of the methodaccording to the invention.

LIST OF REFERENCE NUMBERS

-   1 Thermal treatment chamber-   2 After-incineration chamber-   3 Double-walled graphite scrubber-   4 Single-walled graphite scrubber-   5 Alkaline scrubber-   6 Exhaust gas conduit

1. Ashing plant for enriching noble metals from fluorine-containing materials, comprising a thermal treatment chamber (1) having a refractory insulating lining on the inside of the thermal treatment chamber (1), and an exhaust gas cleaning system, whereby the insulating lining is resistant to hydrofluoric acid and the exhaust gas cleaning system comprises at least one or more acid scrubber(s) (3, 4) and at least one alkaline scrubber (5).
 2. The ashing plant according to claim 1, wherein the exhaust gas cleaning system further comprises at least one or more thermal after-incineration chambers (2).
 3. The ashing plant according to claim 2, wherein the inside of the at least one thermal after-incineration chamber (2) is provided with a hydrofluoric acid-resistant refractory insulating lining.
 4. The ashing plant according to claim 1 wherein the plant further comprising an exhaust gas conduit (6) for guiding the exhaust gases out of the thermal treatment chamber (1) and in that the inside of said exhaust gas conduit (6) is provided with a hydrofluoric acid-resistant refractory insulating lining.
 5. The ashing plant according to claim 1 wherein the refractory insulating lining has an aluminium oxide (Al₂O₃) content of 85% by weight or more.
 6. The ashing plant according to claim 1 wherein the thermal treatment chamber (1), after-incineration chamber (2), and exhaust gas conduit (6) comprise different insulating lining(s).
 7. The ashing plant according to claim 1 wherein thermal treatment chamber (1), after-incineration chamber (2) and/or exhaust gas conduit (6) further comprise an external lining.
 8. The ashing plant according to claim 7, wherein the external lining is a mineral fibre.
 9. The ashing plant according to claim 1 wherein the exhaust gas cleaning system comprises at least one scrubber made of graphite (3) with a double-walled design.
 10. The ashing plant according to claim 9, wherein the double-walled scrubber (3) comprises a water cooling system.
 11. The ashing plant according to claim 1 wherein the exhaust gas cleaning system comprises at least one single-walled scrubber (4) made of graphite.
 12. The ashing plant according claim 1 wherein the exhaust gas cleaning system comprises a double-walled scrubber made of graphite (3) and a single-walled scrubber made of graphite (4).
 13. The ashing plant according to claim 1 wherein the alkaline scrubber (5) comprises on its inside a coating made of a plastic material, in particular made of polypropylene.
 14. The ashing plant according to claim 1 further comprising a control unit.
 15. Method for enriching noble metals from fluorine-containing materials, comprising a thermal treatment of the materials in a thermal treatment chamber (1) having a hydrofluoric acid-resistant refractory insulating lining, and a cleaning of the exhaust gases generated during the thermal treatment, whereby the cleaning comprises the following steps in the following order: a) if applicable, thermal after-incineration in an after-incineration chamber (2), b) scrubbing of the exhaust gases with water and/or an acid, and c) scrubbing of the exhaust gases with a base.
 16. The method according to claim 15 wherein step b) comprises the following steps: b1) scrubbing and simultaneous cooling of the exhaust gases in a double-walled graphite scrubber (3) followed by b2) scrubbing of the exhaust gases in a single-walled graphite scrubber (4).
 17. The method according to claim 15, wherein the salt-enriched scrubbing water obtained in step b) can be fed to a waste water treatment plant periodically or continuously as side stream.
 18. The method according to claim 15 wherein the exhaust gases are washed in step c) with a base with a pH of at least 10 or more.
 19. The method according to claim 15 wherein materials having a fluorine content of up to 5% by weight are used as materials to be subjected to the thermal treatment.
 20. The method according to claim 15 wherein fluoro-organic materials, PTFE films, fuel cells, catalysts and/or pastes are used as materials to be subjected to the thermal treatment.
 21. The method according to claim 15 wherein the method is implemented in an ashing plant comprising a thermal treatment chamber (1) having a refractory insulating lining on the inside of the thermal treatment chamber (1), and an exhaust gas cleaning system, whereby the insulating lining is resistant to hydrofluoric acid and the exhaust gas cleaning system comprises at least one or more acid scrubber(s) (3, 4) and at least one alkaline scrubber (5).
 22. (canceled)
 23. (canceled)
 24. (canceled) 